This invention relates generally to luminance signal peaking circuits and particularly to a common and differential mode amplifier useful in such circuits. In television, a so-called luminance signal is one that is representative of black and white content of an image. It is a baseband signal, developed in the output of the video detector and used to modulate the receiver cathode ray tube to control the brightness and contrast of the reproduced image. In color receivers, the luminance signal is very similar to the complete video signal in monochrome receivers. The luminance channel in a color receiver includes one or more stages of signal amplification and also incorporates a time delay to compensate for the slower processing experienced by the color or chrominance signal in the narrower bandwidth chrominance channel.
While the luminance channel is, comparatively speaking, wideband, analog signals that carry luminance information are very often degraded because of bandwidth limitations. A major type of degradation results in the signal "rise and fall" times being undesirably long with the result that the reproduced picture lacks "crispness". It is well known and conventional to "peak" the luminance signal by adding "pre-shoot" and "overshoot" to effectively speed up its rise and fall times to provide a more pleasing television image.
Many signal peaking methods have been used in the prior art. In general tradeoffs must be made between sharply peaked signals and noise. In U.S. Pat. No. 4,263,616 mentioned above, the peaking methods of "straight coring" and "nonlinear amplification" are discussed. Both methods derive a peaking signal by subtracting the second derivative of a low pass video signal from the video signal itself. The improved method of signal peaking taught in the patent involves developing an additional adjustable control signal having magnitude variations which are related to, and occur in time correlation with, amplitude transitions in both the video signal and the peaking signal. The control signal modulates the peaking signal such that it experiences a first gain at a threshold level and a second gain below that threshold level. The result is television pictures having sharply peaked high level brightness transitions which are still natural looking without excessive noise. This desirable result is due to the presence of a small amount of low level peaking components and noise in the peaking signal.
U.S. Pat. No. 4,296,435, discloses a novel arrangement of the elements in the signal peaking circuitry of U.S. Pat. No. 4,263,616. In particular the series of cascaded signal processing stages, which had been operated from their own sources of DC potential, are replaced by a circuit arrangement which, not only conserves operating power, but includes dynamic noise processed peaking that accentuates luminance transitions. Also, the circuitry does not accentuate noise signal spikes occurring during flat field portions of the luminance signal. Further, the system minimizes "blooming" due to excessive white-going peaking signals and enables the amount of peaking and the brightness or contrast of the reproduced image to be independently controllable by the viewer.
This invention embraces the teachings and circuitry of both of the above patents and provides an improved common and differential mode amplifier for developing the two peaking signals for use by the peaking precessors therein. In particular, U.S. Pat. No. 4,296,435 includes a well-known common and differential mode amplifier for developing the opposite polarity differential mode signals and the like polarity common mode signals. The amplifier is supplied with a common mode signal, comprising the low pass luminance signal, and the current mode signals developed in its two outputs thus include in-phase low pass luminance components. The differential mode signal supplied to the amplifier comprises the second derivative of the low pass luminance signal and the current mode signals developed in the outputs also include out-of-phase second derivative components. The signals in the two outputs are matrixed for further processing with the differential mode gain of the peaking processor being controllable by the viewer.
The prior art differential type amplifier used has a common mode gain and a differential mode gain associated therewith. While the system of the patents work quite well, it would be most desirable to keep the common mode gain and the differential mode gain of the amplifier independent of each other to avoid changes in peaking as a function of signal level changes. Specifically, as the common mode signal level changed, the amount of peaking would change, which was not always desirable.